NOMMU: Make mmap allocation page trimming behaviour configurable.

NOMMU mmap allocates a piece of memory for an mmap that's rounded up in size to
the nearest power-of-2 number of pages.  Currently it then discards the excess
pages back to the page allocator, making that memory available for use by other
things.  This can, however, cause greater amount of fragmentation.

To counter this, a sysctl is added in order to fine-tune the trimming
behaviour.  The default behaviour remains to trim pages aggressively, while
this can either be disabled completely or set to a higher page-granular
watermark in order to have finer-grained control.

vm region vm_top bits taken from an earlier patch by David Howells.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Mike Frysinger <vapier.adi@gmail.com>

1 files changed, 42 insertions, 23 deletions

diff --git a/mm/nommu.c b/mm/nommu.c
index 0d363dfcf10e..a6e8ccfbd400 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -10,7 +10,7 @@
  *  Copyright (c) 2000-2003 David McCullough <davidm@snapgear.com>
  *  Copyright (c) 2000-2001 D Jeff Dionne <jeff@uClinux.org>
  *  Copyright (c) 2002      Greg Ungerer <gerg@snapgear.com>
- *  Copyright (c) 2007      Paul Mundt <lethal@linux-sh.org>
+ *  Copyright (c) 2007-2008 Paul Mundt <lethal@linux-sh.org>
  */
 
 #include <linux/module.h>
@@ -66,6 +66,7 @@ atomic_long_t vm_committed_space = ATOMIC_LONG_INIT(0);
 int sysctl_overcommit_memory = OVERCOMMIT_GUESS; /* heuristic overcommit */
 int sysctl_overcommit_ratio = 50; /* default is 50% */
 int sysctl_max_map_count = DEFAULT_MAX_MAP_COUNT;
+int sysctl_nr_trim_pages = 1; /* page trimming behaviour */
 int heap_stack_gap = 0;
 
 atomic_t mmap_pages_allocated;
@@ -455,6 +456,8 @@ static noinline void validate_nommu_regions(void)
         last = rb_entry(lastp, struct vm_region, vm_rb);
         if (unlikely(last->vm_end <= last->vm_start))
                 BUG();
+        if (unlikely(last->vm_top < last->vm_end))
+                BUG();
 
         while ((p = rb_next(lastp))) {
                 region = rb_entry(p, struct vm_region, vm_rb);
@@ -462,7 +465,9 @@ static noinline void validate_nommu_regions(void)
 
                 if (unlikely(region->vm_end <= region->vm_start))
                         BUG();
-                if (unlikely(region->vm_start < last->vm_end))
+                if (unlikely(region->vm_top < region->vm_end))
+                        BUG();
+                if (unlikely(region->vm_start < last->vm_top))
                         BUG();
 
                 lastp = p;
@@ -536,7 +541,7 @@ static void free_page_series(unsigned long from, unsigned long to)
 /*
  * release a reference to a region
  * - the caller must hold the region semaphore, which this releases
- * - the region may not have been added to the tree yet, in which case vm_end
+ * - the region may not have been added to the tree yet, in which case vm_top
  *   will equal vm_start
  */
 static void __put_nommu_region(struct vm_region *region)
@@ -547,7 +552,7 @@ static void __put_nommu_region(struct vm_region *region)
         BUG_ON(!nommu_region_tree.rb_node);
 
         if (atomic_dec_and_test(&region->vm_usage)) {
-                if (region->vm_end > region->vm_start)
+                if (region->vm_top > region->vm_start)
                         delete_nommu_region(region);
                 up_write(&nommu_region_sem);
 
@@ -558,7 +563,7 @@ static void __put_nommu_region(struct vm_region *region)
                  * from ramfs/tmpfs mustn't be released here */
                 if (region->vm_flags & VM_MAPPED_COPY) {
                         kdebug("free series");
-                        free_page_series(region->vm_start, region->vm_end);
+                        free_page_series(region->vm_start, region->vm_top);
                 }
                 kmem_cache_free(vm_region_jar, region);
         } else {
@@ -999,6 +1004,10 @@ static int do_mmap_shared_file(struct vm_area_struct *vma)
         int ret;
 
         ret = vma->vm_file->f_op->mmap(vma->vm_file, vma);
+        if (ret == 0) {
+                vma->vm_region->vm_top = vma->vm_region->vm_end;
+                return ret;
+        }
         if (ret != -ENOSYS)
                 return ret;
 
@@ -1027,11 +1036,14 @@ static int do_mmap_private(struct vm_area_struct *vma,
          */
         if (vma->vm_file) {
                 ret = vma->vm_file->f_op->mmap(vma->vm_file, vma);
-                if (ret != -ENOSYS) {
+                if (ret == 0) {
                         /* shouldn't return success if we're not sharing */
-                        BUG_ON(ret == 0 && !(vma->vm_flags & VM_MAYSHARE));
-                        return ret; /* success or a real error */
+                        BUG_ON(!(vma->vm_flags & VM_MAYSHARE));
+                        vma->vm_region->vm_top = vma->vm_region->vm_end;
+                        return ret;
                 }
+                if (ret != -ENOSYS)
+                        return ret;
 
                 /* getting an ENOSYS error indicates that direct mmap isn't
                  * possible (as opposed to tried but failed) so we'll try to
@@ -1051,23 +1063,25 @@ static int do_mmap_private(struct vm_area_struct *vma,
         if (!pages)
                 goto enomem;
 
-        /* we allocated a power-of-2 sized page set, so we need to trim off the
-         * excess */
         total = 1 << order;
         atomic_add(total, &mmap_pages_allocated);
 
         point = rlen >> PAGE_SHIFT;
-        while (total > point) {
-                order = ilog2(total - point);
-                n = 1 << order;
-                kdebug("shave %lu/%lu @%lu", n, total - point, total);
-                atomic_sub(n, &mmap_pages_allocated);
-                total -= n;
-                set_page_refcounted(pages + total);
-                __free_pages(pages + total, order);
+
+        /* we allocated a power-of-2 sized page set, so we may want to trim off
+         * the excess */
+        if (sysctl_nr_trim_pages && total - point >= sysctl_nr_trim_pages) {
+                while (total > point) {
+                        order = ilog2(total - point);
+                        n = 1 << order;
+                        kdebug("shave %lu/%lu @%lu", n, total - point, total);
+                        atomic_sub(n, &mmap_pages_allocated);
+                        total -= n;
+                        set_page_refcounted(pages + total);
+                        __free_pages(pages + total, order);
+                }
         }
 
-        total = rlen >> PAGE_SHIFT;
         for (point = 1; point < total; point++)
                 set_page_refcounted(&pages[point]);
 
@@ -1075,6 +1089,7 @@ static int do_mmap_private(struct vm_area_struct *vma,
         region->vm_flags = vma->vm_flags |= VM_MAPPED_COPY;
         region->vm_start = (unsigned long) base;
         region->vm_end   = region->vm_start + rlen;
+        region->vm_top   = region->vm_start + (total << PAGE_SHIFT);
 
         vma->vm_start = region->vm_start;
         vma->vm_end   = region->vm_start + len;
@@ -1110,6 +1125,7 @@ error_free:
         free_page_series(region->vm_start, region->vm_end);
         region->vm_start = vma->vm_start = 0;
         region->vm_end   = vma->vm_end = 0;
+        region->vm_top   = 0;
         return ret;
 
 enomem:
@@ -1401,7 +1417,7 @@ int split_vma(struct mm_struct *mm, struct vm_area_struct *vma,
         npages = (addr - vma->vm_start) >> PAGE_SHIFT;
 
         if (new_below) {
-                region->vm_end = new->vm_end = addr;
+                region->vm_top = region->vm_end = new->vm_end = addr;
         } else {
                 region->vm_start = new->vm_start = addr;
                 region->vm_pgoff = new->vm_pgoff += npages;
@@ -1418,6 +1434,7 @@ int split_vma(struct mm_struct *mm, struct vm_area_struct *vma,
                 vma->vm_region->vm_pgoff = vma->vm_pgoff += npages;
         } else {
                 vma->vm_region->vm_end = vma->vm_end = addr;
+                vma->vm_region->vm_top = addr;
         }
         add_nommu_region(vma->vm_region);
         add_nommu_region(new->vm_region);
@@ -1454,10 +1471,12 @@ static int shrink_vma(struct mm_struct *mm,
 
         down_write(&nommu_region_sem);
         delete_nommu_region(region);
-        if (from > region->vm_start)
-                region->vm_end = from;
-        else
+        if (from > region->vm_start) {
+                to = region->vm_top;
+                region->vm_top = region->vm_end = from;
+        } else {
                 region->vm_start = to;
+        }
         add_nommu_region(region);
         up_write(&nommu_region_sem);